1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus provided with a development device.
2. Description of the Related Art
Generally, in an electrophotographic image forming apparatus, an image is formed through an image forming process. The image forming process includes a charging process, an exposure process, a development process, a transfer process, a fixing process, and a cleaning process.
In the image formation, a surface of an electrophotographic photosensitive body (hereinafter referred to as a “photosensitive body”) is evenly charged. Then the photosensitive body is exposed according to image information to form an electrostatic latent image. When toner is supplied to the electrostatic latent image, the electrostatic latent image becomes a toner image. Then the toner image formed on the photosensitive body is transferred onto a recording material such as a sheet. A fixing process heats and pressurizes the recording material to which the toner image is transferred, thereby fixing the toner image to the surface of the recording material. Therefore, the image formation to the recording material is ended. On the other hand, transfer residual toner remains on the surface of the photosensitive body after the toner image is transferred. The transfer residual toner is removed through the cleaning process.
Conventionally, a two-component development system in which non-magnetic toner (toner) and a magnetic carrier (carrier) are mixed and used as a developer is widely spread in the electrophotographic image forming apparatus, particularly in the image forming apparatus that forms a color image. Compared with other currently-well-known development systems, the two-component development system has advantages, such as stability of image quality and durability of the apparatus.
In the image forming apparatus in which the two-component development system is used, generally the following sequence is performed when the toner image is formed by developing the electrostatic latent image formed on the photosensitive drum that is of an image bearing member. First the surface of the photosensitive drum is evenly charged by a charging portion so as to become a white-background-part potential Vd. A development bias is applied to a development sleeve that is of a developer bearing member, and the development sleeve is set to a potential identical to a direct-current component Vdc of the development bias.
At this point, a potential difference between the white-background-part potential Vd and the direct-current component Vdc of the development bias is set so as to become a desired fog removing potential difference Vback. An image part (development part) on the photosensitive drum is exposed by an exposure portion that forms the electrostatic latent image, thereby becoming an attenuated bright-part potential VL. The toner on the development sleeve moves to the photosensitive drum by a contrast potential difference Vcont that is of a difference with the direct-current component Vdc of the development bias. Thus, the electrostatic latent image formed on the photosensitive drum is developed as the toner image.
Generally, in the two-component development system, when the toner is consumed because of the image formation, the toner is replenished according to the consumed toner. Therefore, the toner in the developer is sequentially replaced by repeating the image formation.
However, the following problem is generated in the image forming apparatus in which the two-component development system is adopted.
Nowadays, various sheet types are used as the recording material. Examples of the recording material include inexpensive paper (such as recycled paper) having low surface smoothness, paper having high smoothness, and coated paper. Among them, for the paper having the low smoothness, a toner transfer characteristic is changed along a shape of a paper surface, and uneven transfer is easily generated.
Generally a transfer condition (transfer bias) is changed according to a setting of a sheet type (such as plain paper, recycled paper, thick paper, and OHT) that is selected by a user. However, when the toner is degraded due to lasting, a temperature rise and the like, it is difficult to maintain the transfer characteristic to the paper through the lasting.
On the other hand, generally the surface of the toner is covered with an additive (hereinafter referred to as an external additive) in order to maintain fluidity of the toner. The external additive is a particle derived from alumina or silica, and the external additive has particle diameters of tens to hundreds of nanometers. The fluidity is provided to the toner when the external additive is added, so that the toner can efficiently be transferred to the paper.
However, generally the shape and particle diameter of the toner vary. Therefore, an external additive coverage per one particle of the toner also varies. While a certain toner particle is stably covered with the external additive, sometimes another toner particle is covered with a small amount of external additive.
In the case that the toner covered with a small amount of external additive is supplied, when the image is formed on the paper having the low smoothness, the toner covered with a small amount of external additive is insufficiently transferred, and possibly the uneven transfer is generated. For example, in the case that the toner covered with a small amount of external additive is included, it is difficult to maintain the transfer characteristic to various sheet types.
When the image forming operation is performed for a long period of time, sometimes a decrease of an adhesion amount of the external additive is generated. Specifically, when the image (an image having a low print coverage) in which toner consumption amount is small is continuously output, a small amount of toner is replaced in the developer. As a result, the toner that is not replaced exists in the development device for a long time, the toner that is not replaced circulates in the development device for a long period of time. When the toner exists in the development device for a long time, the toner is repeatedly slid and agitated, and the external additive is buried in the surface of the toner. Therefore, there is a risk of degrading the fluidity of the developer. When the fluidity of the developer is degraded, adhesion between the toner and the image bearing member is increased to decrease the transfer characteristic. Particularly, the transfer characteristic to the paper having the low smoothness is degraded.
For example, Japanese Patent Laid-Open No. 2000-310909 discloses a technology of suppressing the degradation of the fluidity of the toner. In Japanese Patent Laid-Open No. 2000-310909, a unit calculates a print coverage of the formed image, and the toner is forcedly consumed by developing a predetermined amount of toner in a non-image region when the calculated print coverage is less than a predetermined value. The new toner corresponding to the consumed toner is replenished to the development device, and the degraded toner is replaced with the new toner. Performing such a control prevents the uneven transfer, which is generated by not replacing the toner in the development device when the image having the low print coverage is continuously output.
In Japanese Patent Laid-Open No. 2000-310909, it is expected that a fixed effect is obtained when the transfer defect is generated because the toner is degraded by the use of the image forming apparatus. However, originally the external additive coverage per one particle of the toner varies as described above. Therefore, the developed toner includes the toner having a small amount of external additive. In this case, when the low-smoothness paper, such as an embossed paper, is used, sometimes the uneven transfer is generated even if the toner is replaced.